1. Field of the Invention
The present invention relates to a decoding circuit for decoding an encoded bit stream produced from an audio signal and a reproduction apparatus including the same.
2. Description of hte Related Art
An encoded bit stream produced in an audio signal encoding system generally includes an encoded audio signal and header information, which is decoding information used for decoding the encoded audio signal. The header information includes decoding parameters such as, for example, sampling frequency information, emphasis information, number of channels, and channel mode information. Such an encoded bit stream is produced by, for example, MPEG1 and MPEG2 audio signal encoding systems (see, ISO/IEC 11172-3:1993 and ISO/IEC CD 13818-3.2).
A reproduction apparatus for reproducing such an encoded bit stream generally includes a decoding circuit for decoding the encoded bit stream and thus generating a digital output signal, a D/A converter for converting the digital output signal into an analog signal, peripheral circuits such as, for example, a medium data extraction circuit for transferring the encoded bit stream from the medium storing the encoded bit stream to the decoding circuit, and a CPU for performing overall control of the reproduction apparatus.
FIG. 5 is a block diagram illustrating a structure of a reproduction apparatus 500 for reproducing an MPEG1 encoded audio signal. The reproduction apparatus 500 operates in the following manner.
An MPEG1 audio bit stream extracted from a medium data extraction circuit 531 is sent to a stream buffer 532 by the control of a CPU 533. Upon an instruction from the CPU 533, a decoding circuit 534 starts decoding the audio bit stream. The decoding circuit 534 operates in the following manner.
The audio bit stream is read by a stream reading section 342. A header analysis section 343 extracts header information from the audio bit stream and performs header analysis, thereby obtaining decoding parameters such as, for example, sampling frequency information, layer information and channel mode information.
The sampling frequency information obtained by the header analysis section 343 is sent to the CPU 533 through a host interface section 341, and the CPU 533 sets a D/A converter 536.
The decoding circuit 534, after header analysis, decodes the encoded audio signal by a signal processing section 344, based on the decoding parameters obtained by the header analysis section 343, into a digital output signal. The resultant digital output signal is sent to an output buffer 535 by an output section 345. The digital output signal temporarily stored in the output buffer 535 is sequentially read and converted into an analog signal by the D/A converter 536, which is output outside.
The reproduction apparatus 500 has the following problems.
(1) When the sampling frequency is changed during reproduction, the sampling frequency for the D/A converter 536. needs to be reset. However, it is very difficult to reset the sampling frequency in synchronization with the decoding processing performed by the decoding circuit 534. The reasons are that resetting of the D/A converter 536 is time-consuming and that there is a delay from the change in the sampling frequency until the CPU 533 is informed of the change in the sampling frequency.
Such a change in the decoding parameters can actually occur as a result of a change in the track or channel of the reproduction stream. In the decoding circuit and reproduction apparatus as described above, an extraneous or discordant sound (noise) is output when such a change in the decoding parameters occurs.
(2) For reproducing an encoded bit stream packetized with time stamp information resulting from the DVD format or STB (set top box) format used, the bit stream needs to be decoded in synchronization with the time stamp information into a digital output signal to be output.
When the encoded bit stream to be decoded is, for example, an MPEG encoded audio stream, the number of output data samples per frame-and the reproduction time vary in accordance with the decoding parameters included in the header information, such as, for example, sampling frequency information, layer information, and ID information indicating whether or not half rate sampling is performed.
When such decoding parameters change during reproduction and the encoded bit stream continues to be decoded in synchronization with the time stamp information without resetting the internal decoding processing, an extraneous or discordant sound is output.
According to one aspect of the invention, a decoding circuit, for receiving a bit stream including an encoded audio signal and header information used for decoding the encoded audio signal, and decoding the encoded audio signal based on the header information, includes a header analysis section for outputting at least one decoding parameter obtained from the header information and decoding parameter change information indicating whether or not the at least one decoding parameter has been changed; a signal processing section for decoding the encoded audio signal, based on the at least one decoding parameter, into a decoded signal and outputting the decoded signal; an automatic mute processing section for executing automatic mute on the decoded signal after the at least one decoding parameter is changed; and an output section for outputting the decoded signal output from the automatic mute processing section.
In one embodiment of the invention, the automatic mute processing section executes the automatic mute for a prescribed duration of time.
In one embodiment of the invention, the decoding circuit further includes a host interface section for communicating with a control circuit for controlling the decoding circuit.
In one embodiment of the invention, the host interface section includes an automatic mute instruction section which is set to instruct whether or not automatic mute is necessary.
In one embodiment of the invention, the at least one decoding parameter includes sampling frequency information. The decoding parameter change information is sampling frequency change information indicating whether or not the sampling frequency information has been changed. The automatic mute processing section executes the automatic mute based on the sampling frequency change information. The automatic mute processing instruction section includes a sampling frequency change automatic mute instruction section which is set to instruct whether or not the automatic mute is necessary based on the change in the sampling frequency information.
In one embodiment of the invention, the at least one decoding parameter includes emphasis information. The decoding parameter change information is emphasis change information indicating whether or not the emphasis information has been changed. The automatic mute processing section executes the automatic mute based on the emphasis change information. The automatic mute processing instruction section includes an emphasis change automatic mute instruction section which is set to instruct whether or not the automatic mute is necessary based on the change in the emphasis information.
In one embodiment of the invention, the at least one decoding parameter includes channel mode information. The decoding parameter change information is channel mode change information indicating whether or not the channel mode information has been changed. The automatic mute processing section executes the automatic mute based on the channel mode change information. The automatic mute processing instruction section includes a channel mode change automatic mute instruction section which is set to instruct whether or not the automatic mute is necessary based on the change in the channel mode information.
In one embodiment of the invention, the bit stream includes an MPEG encoded audio bit stream.
In one embodiment of the invention, the at least one decoding parameter includes layer information. The decoding parameter change information is layer change information indicating whether or not the layer information has been changed. The automatic mute processing section executes the automatic mute based on the layer change information. The automatic mute processing instruction section includes a layer change automatic mute instruction section which is set to instruct whether or not the automatic mute is necessary based on the change in the layer information.
In one embodiment of the invention, the at least one decoding parameter includes ID information. The decoding parameter change information is ID change information indicating whether or not the ID information has been changed. The automatic mute processing section executes the automatic mute based on the ID change information. The automatic mute processing instruction section includes an ID change automatic mute instruction section which is set to instruct whether or not the automatic mute is necessary based on the change in the ID information.
In one embodiment of the invention, the decoding circuit further includes an initialization section for automatically initializing decoding processing performed by the decoding circuit, wherein the host interface section includes an initialization instruction section for instructing the initialization section to initialize the decoding processing upon receipt of an instruction from the control circuit.
In one embodiment of the invention, the host interface section includes a parameter display section for outputting the at least one decoding parameter to the control circuit, and a parameter change interrupt section for outputting the decoding parameter change information to the control circuit.
In one embodiment of the invention, the host interface section includes an automatic mute timer setting section for setting a time duration. The automatic mute processing section terminates the automatic mute based on an instruction from the automatic mute timer setting section.
In one embodiment of the invention, the host interface section includes an automatic mute release instruction section which is set by the control circuit whether or not to instruct automatic mute release. The automatic mute processing section terminates the automatic mute based on an instruction from the automatic mute release instruction section.
In one embodiment of the invention, the at least one decoding parameter includes sampling frequency information. The decoding parameter change information is sampling frequency change information indicating whether or not the sampling frequency information has been changed. The automatic mute processing section starts the automatic mute based on the sampling frequency change information.
In one embodiment of the invention, the at least one decoding parameter includes emphases information. The decoding parameter change information is emphasis change information indicating whether or not the emphases information has been changed. The automatic mute processing section starts the automatic mute based on the emphasis change information.
In one embodiment of the invention, the at least one decoding parameter includes channel mode information. The decoding parameter change information is channel mode change information indicating whether or not the channel mode information has been changed. The automatic mute processing section starts the automatic mute based on the channel mode change information.
In one embodiment of the invention, the bit stream includes an MPEG encoded audio bit stream.
In one embodiment of the invention, the at least one decoding parameter includes layer information. The decoding parameter change information is layer change information indicating whether or not the layer information has been changed. The automatic mute processing section starts the automatic mute based on the layer change information.
In one embodiment of the invention, the decoding parameter includes ID information. The decoding parameter change information is ID change information indicating whether or not the ID information has been changed. The automatic mute processing section starts the automatic mute based on the ID change information.
According to another aspect of the invention, a reproduction apparatus includes the above-described decoding circuit, and a control circuit for controlling the decoding circuit.
In one embodiment of the invention, the reproduction apparatus further includes a medium data extraction circuit for extracting an encoded audio signal and header information from an input encoded audio bit stream and outputting the encoded audio signal and the header information to the decoding circuit.
In one embodiment of the invention, the reproduction apparatus further includes a D/A converter for D/A converting a decoded signal output from the decoding circuit.
In one embodiment of the invention, the control circuit stops an operation of the medium data extraction circuit based on decoding parameter change information obtained from the header information.
In one embodiment of the invention, the control circuit sets a sampling frequency for the D/A converter based on the decoding parameter change information obtained from the header information.
In one embodiment of the invention, the control circuit resets the decoding circuit based on the decoding parameter change information obtained from the header information.
In one embodiment of the invention, the control circuit restarts an operation of the medium data extraction circuit based on the decoding parameter change information obtained from the header information.
Thus, the invention described herein makes possible the advantages of providing a decoding circuit for decoding an encoded bit stream without outputting an extraneous or discordant sound even when a decoding parameter changes during reproduction, and a reproduction apparatus including such a decoding circuit.